Fiber laser amplifiers are used in a variety of applications, such as low-power marking and cutting, high-power industrial cutting and welding, and high-power defense oriented applications. Fiber lasers can be particularly useful for processing materials that are difficult to process with traditional machining methods. For example, fiber lasers can be used to cut Gorilla Glass™, which is widely used in cell phones. Fiber lasers can be also useful in welding components that may not withstand large quantities of heat. In this case, localized heating by the laser performs the welding without damaging adjacent material.
FIG. 1 shows a schematic of a conventional fiber laser amplifier 100. The amplifier 100 includes an isolator 110 to receive an input signal 105 (e.g., provided by a laser oscillator; now shown in FIG. 1), a gain fiber 120 to amplify the input signal 105, and pump diodes 130 to pump the gain fiber 120. The isolator 110 can transmit the input signal 105 along the forward direction (i.e., from the isolator 110 to the gain fiber 120) but block any input signal 105 propagating along the backward direction (i.e., from the gain fiber 120 to the isolator 110). This prevents the amplified input signal from damaging the source of the input signal 105, such as a laser oscillator. The pump diodes 130 typically pump the cladding of the gain fiber 120. However, for low power applications, core pumping may also be used to reduce or minimize the length of the gain fiber 120.
The amplifier 100 can be operated at high gain (e.g., >17 dB), and there is usually large population inversion at the beginning of the gain fiber 120. At least two adverse side effects can be induced from this large population inversion. First, photodarkening can occur at the beginning of the gain fiber 120, which can reduce the efficiency of the amplifier 100 and increase the heat load. The heat load can be especially problematic in high power amplifiers. Second, significant amplified spontaneous emission (ASE) can be generated in the gain fiber 120, thereby reducing the efficiency of the amplifier 100. The backward propagating ASE can also cause component failure at the input of the fiber amplifier 100. For example, the ASE propagating back to the isolator 110 can damage the isolator 110, which is usually costly.